The vast majority of electronic cameras have autofocus capability. The autofocus function automatically focuses the camera on objects in the scene viewed by the camera. Autofocus may be fully automatic such that the camera identifies objects in the scene and focuses on the objects. In some cases, the camera may even decide which objects are more important than other objects and subsequently focus on the more important objects. Alternatively, autofocus may utilize user input specifying which portion or portions of the scene are of interest. Based thereupon, the autofocus function identifies objects within the portion or portions of the scene, specified by the user, and focuses the camera on such objects.
To achieve market adoption, the autofocus function must be reliable and fast such that every time a user captures an image, the camera quickly brings the desired portion, or portions, of the scene into focus. Preferably, the autofocus function is sufficiently fast that the user does not notice any delay between pressing the trigger button and image capture. The autofocus is particularly important for cameras having no means for manual focus, such as compact digital cameras and camera phones.
Many electronic cameras use contrast autofocus, wherein the autofocus function adjusts the imaging objective to maximize contrast in at least a portion of the scene, thus bringing the portion of the scene into focus. More recently, phase-detection autofocus has gained popularity because it is faster than contrast autofocus. Phase-detection autofocus directly measures the degree of misfocus by comparing light passing through one portion of the imaging objective, e.g., the left portion, with light passing through another portion of the imaging objective, e.g., the right portion. Some digital single-lens reflex cameras include a dedicated phase-detection sensor in addition to the image sensor that captures images.
However, this solution is not feasible for more compact and/or less expensive cameras. Therefore, camera manufacturers are developing image sensors with on-chip phase detection, i.e., image sensors with integrated phase detection capability. A variety of pixel layouts have been proposed for this purpose. These include pixel layouts that include phase-detection pixels that have an opaque mask (or equivalently a shield) that blocks light from reaching one side of the pixel. Such partially-masked (partially-shielded) pixels enable distinguishing light passing through the imaging objective from different directions. Drawbacks of this approach are that the light-blocking masks decrease pixel sensitivity in low-light conditions, shadow neighboring pixels, and reflect light that results in cross-talk in neighboring pixels.